1. Field of the Invention
The present invention relates to improved valley-mode switching schemes for switch-mode power converters.
2. Description of the Related Art
Each time a switch, such as a transistor, is turned on or off, energy is dissipated in proportion to the current and voltage being switched. The power losses associated with switch operation, referred to as switching losses, represent a significant source of power dissipation and therefore a significant source of inefficiency within conventional switch-mode power converters. In addition to increasing switching loss, large rates of change for voltages and/or currents (i.e., dv/dt and/or di/dt) at the time of switch transition increase stress upon the switch and the amount of electromagnetic interference (EMI) produced by the switch. Some switching schemes have been developed that take advantage of resonance within switch-mode power converters to turn on switches at times when the voltages applied to the switches are at a local minimum referred to as a valley. Such switching schemes are therefore typically referred to as valley-mode switching schemes (VMS schemes).
However, conventional VMS schemes are inadequate for several reasons. The resonant characteristics of a switch-mode power converter fluctuate in time based on operating conditions. For example, fluctuations in an input line voltage or in an output current drawn from the switch-mode power converter by an external load can affect the resonant behavior of the switch-mode power converter and therefore affect timing for valleys. Because conventional VMS schemes typically turn on the switch of the switch-mode power converter at the first valley in a switching cycle, such fluctuations in resonance therefore precipitate changes in switching frequency. These changes in switching frequency result in increased EMI emissions and make conventional VMS schemes compatible only with control schemes that utilize a variable switching frequency such as pulse frequency modulation (PFM) control schemes. For example, power converters employing conventional VMS schemes typically exhibit switching frequencies as low as 40 kHz and as high as 130 kHz mainly for the sole purpose of achieving VMS turn on. Furthermore, conventional VMS schemes require significant fundamental modification to feedback control schemes used by the switch-mode power converters to regulate signals such as output voltage and output current. Conventional VMS schemes also do not account for losses associated with turning off the power switch. Moreover, the high switching frequencies typically required by conventional VMS schemes (e.g., 130 kHz) make them unsuitable for switch-mode power converters that utilize one or more slow power switches such as bipolar junction transistors (BJTs).